RI-32 Shorter Contributions to the Geology of the Sioux Quartzite (Early Proterozoic), Southwestern Minnesota

Abstract

The lower Proterozoic Sioux Quartzite is exposed at several localities within an east-trending area 300 km long and 50 km wide between Mitchell, South Dakota, and New Ulm, Minnesota. The quartzite unconformably overlies Archean rocks and is overlain by Cretaceous strata and Pleistocene glacial materials. The formation consists predominantly of orthoquartzitic sandstone. Minor quantities of interbedded conglomeratic orthoquartzite occur in the lower two-thirds of the section, and thin beds of mudstone occur in the upper third. The orthoquartzite is compositionally and texturally mature and is composed largely of well-rounded, well-sorted, monocrystalline quartz and lesser amounts of detrital chert and iron-formation. Grains of polycrystalline quartz are abundant at several places, including New Ulm where they were derived from underlying granite, and near Jackson where they occur in the lower half of the exposed section. The framework grains are coated with thin films of iron oxide and indurated by an epitaxial quartz cement. Multicycle grains with abraded quartz overgrowths beneath the latest cement are present in most orthoquartzite thin sections. Rounded zircon and minor rounded tourmaline make up the nonopaque detrital heavy mineral fraction. Two types of conglomeratic rocks are present--coarse basal conglomerate and conglomeratic orthoquartzite. The basal conglomerates at two locali ties contain clasts, as large as 35 cm, of vein quartz, jasper, iron-formation, quartzite, and rhyolite. The conglomeratic orthoquartzite units are characterized by scattered pebble-size clasts of quartz in beds 1 to 10 cm thick. The pebble-rich beds are intercalated with units of cross-bedded, coarse-grained orthoquartzite. Mudstone within the Sioux is red to dark purple in color, ranges from almost pure claystone to silty mudstone, and is composed dominantly of sericite, quartz, and hematite. Trough cross-bedding, planar cross-bedding, symmetrical and asymmetrical ripple marks, and mud cracks are the maj or sedimentary structures. The cross-bedding consists predominantly of narrow troughs 60 to 140 cm wide and 15 to 30 cm thick. Some herringbone cross-bedding is present in the upper third of the section. Measurements of 1, 156 cross-beds have a vector mean of 1620 and a variance of 2,668, implying a paleoslope inclined to the southeast. No major vertical or lateral changes in cross-bedding trends were observed. Analysis of 491 ripple marks also indicates a paleoslope inclined to the southeast. Bimodal-bipolar paleocurrent patterns occur in the upper part of the formation. Most of the Sioux Quartzite is interpreted to have been deposited in a distal braided river-alluvial plain environment. However the upper third of the formation has sedimentary structures that possibly are indicative of a shallow marine, tidally influenced environment. The Sioux is in part a multicycle sediment, with components derived from older quartzite and iron-formation. However, the great volume of monocrystalline quartz must have been chiefly derived from an extensive, deeply weathered, low-relief terrane of dominantly plutonic rocks that also contained some volcanic and metamorphic components. Paleocurrent patterns imply that this terrane existed to the north of the Sioux outcrop area.